vacuumlazy.c: Standardize rel_pages terminology.

vacuumlazy.c: Standardize rel_pages terminology.

VACUUM's rel_pages field indicates the size of the target heap rel just
after the table_relation_vacuum() operation began.  There are specific
expectations around how rel_pages can be related to other nearby state.
In particular, the range of rel_pages must contain every tuple in the
relation whose tuple headers might contain an XID < OldestXmin.

Consistently refer to the field as rel_pages to make this clearer and
more discoverable.

This is follow-up work to commit 73f6ec3d from earlier today.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20220311031351.sbge5m2bpvy2ttxg@alap3.anarazel.de